Amplifiers are used in many circuits, including drivers, to amplify signal voltage and/or current. While amplifiers generally increase signal power, power amplifiers are specifically designed to significantly increase signal power. Drivers typically include one or more power amplifiers to perform work, e.g., to source or sink current to or from a load. Drivers drive signals to loads in a wide variety of applications, from audio signals in audio headsets to data, audio and video signals in communication lines, e.g., phone lines, coaxial cables and Ethernet cables.
Amplifier architecture is impacted by many factors, including capability (e.g. amplification range, signal frequency bandwidth), performance (e.g., signal quality, amplification linearity, noise rejection), manufacturing costs (e.g. die or board area consumption), implementation costs (e.g. additional components required) and operating costs (e.g., power consumption). It is desirable and advantageous to improve amplifier architecture in one or more of the foregoing factors.
Reduced power consumption is generally desirable for many mobile and fixed applications. For example, in mobile devices that rely on batteries, such as cellular telephones, tablets and music players, consumers view longer battery life as a desirable feature, yet high quiescent current reduces battery life. Quiescent current flows when an amplifier is operational, but not performing any work. However, reducing quiescent current typically reduces amplifier capability and/or performance while increasing amplifier implementation costs by requiring additional circuitry, such as switching regulators and expensive external inductors. Accordingly, conventional portable devices generally lack high fidelity audio or long battery life.
Several classes of amplifier exhibit different quiescent current characteristics. For example, a Class-A amplifier consumes current in the absence of a signal, i.e., it has a large quiescent current, which makes it unattractive for low power consumption applications. A Class-B amplifier does not consume current in the absence of a signal, but has cross-over distortion that may degrade its signal quality. A class AB amplifier has less quiescent current than a class A amplifier and less cross-over distortion than a class B amplifier.
A compromise between improved performance and lower quiescent current may be reached by implementing more than one amplifier stage within an amplifier. One technique is to deploy a class AB amplifier stage to provide lower levels of output power to the load and a class B amplifier stage to provide higher levels of output power to the load. However, amplification stage control is difficult. Poor control of thresholds to activate and deactivate amplifier stages may degrade performance by causing undesirable transitions between amplification stages and nonlinear amplification. Good threshold control typically requires additional circuitry, which increases manufacturing and/or implementation costs and also may degrade performance by causing larger parasitic capacitance that impacts stability.
Accordingly, there is a need to overcome the drawbacks and deficiencies in the art by providing a multi-stage amplifier and control circuitry with improved performance to provide smooth transitions between the amplifier stages and linear amplification by overcoming threshold and amplification mismatches between the amplifiers and doing so without increasing or by reducing manufacturing costs, implementation costs and operating costs compared to conventional techniques.